1. Field of the Invention
The present invention relates to a liquid crystal backlight device, and relates more particularly to the drive device for a cold cathode fluorescent lamp using a piezoelectric transformer and used for the backlight device in liquid crystal displays such as used in personal computers, flat panel monitors, and flat panel televisions.
2. Description of Related Art
Piezoelectric transformers achieve extremely high voltage gain when the load is unlimited, and the gain ratio decreases as the load decreases. Other advantages of piezoelectric transformers are that they are smaller than electromagnet transformers, noncombustible, and do not emit noise due to electromagnetic induction. Piezoelectric transformers are used as the power supply for cold cathode fluorescent lamps due to these features.
FIG. 26 shows the configuration of a Rosen-type piezoelectric transformer, a typical piezoelectric transformer according to the prior art. As shown in FIG. 26, this piezoelectric transformer has a low impedance part 510, high impedance part 512, input electrodes 514D and 514U, output electrode 516, and piezoelectric bodies 518 and 520. Reference numeral 522 indicates the polarization direction of the piezoelectric body 518 in the low impedance part 510, reference numeral 524 indicates the polarization direction in piezoelectric body 520, and reference numeral 610 indicates the piezoelectric transformer.
When piezoelectric transformer 610 is used for voltage gain, the low impedance part 510 is the input side. As indicated by polarization direction 522 the low impedance part 510 is polarized in the thickness direction, and input electrodes 514U and 514D are disposed on the primary front and surfaces in the thickness direction. The high impedance part 512 is the output part when the piezoelectric transformer is used for voltage gain. As indicated by polarization direction 524 the high impedance part 512 is polarized lengthwise and has output electrode 516 on the lengthwise end of the transformer.
A specific ac voltage applied between input electrodes 514U and 514D excites a lengthwise expansion and contraction vibration, which piezoelectric effect of the piezoelectric transformer 610 converts to a voltage between input electrode 514U and output electrode 516. Voltage gain or drop results from impedance conversion by the low impedance part 510 and high impedance part 512.
A cold cathode fluorescent lamp with a cold cathode configuration not having a heater for the discharge electrode is generally used for the backlight of a LCD. The striking voltage for starting the lamp and the operating voltage for maintaining lamp output are both extremely high in a cold cathode fluorescent lamp due to the cold cathode design. An operating voltage of 800 Vrms and striking voltage of 1300 Vrms are generally required for a cold cathode fluorescent lamp used in a 14-inch class LCD. As LCD size increases and the cold cathode fluorescent lamp becomes longer, the striking voltage and operating voltage are expected to rise.
FIG. 27 is a block diagram of a self-excited oscillating drive circuit for a prior art piezoelectric transformer. Variable oscillator 616 generates the ac drive signal for driving piezoelectric transformer 610. The variable oscillator 616 generally outputs a pulse wave from which the high frequency component is removed by wave shaping circuit 612 for conversion to a near-sine wave ac signal. Drive circuit 614 amplifies output from wave shaping circuit 612 to a level sufficient to drive the piezoelectric transformer 610. The amplified voltage is input to the primary electrode of piezoelectric transformer 610. The voltage input to the primary electrode is stepped up by the piezoelectric effect of the piezoelectric transformer 610, and removed from the secondary electrode.
The high voltage output from the secondary side is applied to over-voltage protection circuit 630 and the serial circuit formed by cold cathode fluorescent lamp 626 and feedback resistance 624. The over-voltage protection circuit 630 consists of voltage-dividing resistances 628a and 628b, and comparator 620 for comparing the voltages detected at the node between voltage-dividing resistances 628a and 628b with a set voltage. The over-voltage protection circuit 630 controls the oscillation control circuit 618 to prevent the high voltage potential output from the secondary electrode of the piezoelectric transformer from becoming greater than the set voltage. The over-voltage protection circuit 630 does not operate when the cold cathode fluorescent lamp 626 is on.
In the over-voltage protection circuit 630, the voltage occurring at both ends of the feedback resistance 624 is applied to the comparator 620 as a result of the current flowing to the series circuit of cold cathode fluorescent lamp 626 and feedback resistance 624. The comparator 620 compares the set voltage with the feedback voltage, and applies a signal to the oscillation control circuit 618 so that a substantially constant current flows to the cold cathode fluorescent lamp 626. Oscillation control circuit 618 output applied to the variable oscillator 616 causes the variable oscillator 616 to oscillate at a frequency matching the comparator output. The comparator 620 does not operate until the cold cathode fluorescent lamp 626 is on.
Cold cathode fluorescent lamp output is thus stable. This self-exciting drive method enables the drive frequency to automatically track the resonance frequency even when the resonance frequency varies because of the temperature.
This piezoelectric inverter configuration makes it possible to maintain a constant current flow to the cold cathode tube.
As shown in FIG. 23, a method of driving the cold cathode fluorescent lamp by parallel driving two piezoelectric transformers, and a drive method wherein the two output electrodes of the piezoelectric transformers are connected to two input terminals of the cold cathode fluorescent lamp, have been proposed as a way to prevent uneven brightness. The cold cathode fluorescent lamp in these cases is connected as shown in FIG. 25.
Similarly to the drive circuit shown in FIG. 27, these drive circuits also need feedback of current flow to the lamp in order to control the frequency or voltage. It is alternatively possible to detect and feed back the cold cathode fluorescent lamp brightness.
Piezoelectric transformer output current or output voltage is held constant in order to hold the cold cathode fluorescent lamp brightness constant, or current flow to the reflector is detected and fed back for control.
A conventional piezoelectric transformer and drive circuit therefore thus connect a resistance near the cold cathode fluorescent lamp ground and use the voltage of this resistance in order to control the brightness of the cold cathode fluorescent lamp when the cold cathode fluorescent lamp is on. A problem with this method is that uneven brightness occurs as a result of current leaks.
To resolve this problem, Japanese Laid-Open Patent Publication No.11-8087 teaches a means for inputting 180xc2x0 different phase voltages from opposite ends of the cold cathode fluorescent lamp. This configuration is shown in FIG. 22. However, when a cold cathode fluorescent lamp is connected as shown in FIG. 22, current flows to the reflector from the cold cathode fluorescent lamp 330 on the high potential side, and current flows from the reflector to the cold cathode fluorescent lamp on the low potential side. Piezoelectric transformer output current thus contains both current flowing to the lamp and current flowing to a parasitic capacitance. As a result, the output current detection circuit 344 in the drive circuit of a piezoelectric transformer 340 configured as shown in FIG. 25 thus detects both the current flowing to the cold cathode fluorescent lamp 346 and the leakage current of the parasitic capacitance 348 consisting of cold cathode fluorescent lamp 346 and reflector 350. If the parasitic capacitance 348 of the reflector 350 is constant, this constant parasitic capacitance can be taken into consideration to keep current flow to the cold cathode fluorescent lamp 346 constant. However, the parasitic capacitance 348 varies, the leakage current varies with the drive frequency, and it is therefore difficult in practice to maintain a constant current flow to the cold cathode fluorescent lamp 346. The drive circuit shown in FIG. 23 having two piezoelectric transformers also has this problem.
To address this problem, Japanese Laid-open Patent Publication No.11-27955 teaches a method for controlling lamp current by detecting leakage current with a parasitic capacitance current detection circuit, and detecting lamp current with a lamp current detection circuit. In a piezoelectric transformer that controls the drive frequency to maintain constant output using this method, however, the impedance will vary due to the parasitic capacitance if the leakage current frequency varies due to parasitic capacitance, or the parasitic capacitance varies with the unit. The leakage current thus varies. The circuit design must therefore consider both frequency and the effects of the unit, and the control circuit thus becomes more complex.
Furthermore, the cold cathode fluorescent lamp must be connected in series because the secondary terminal of the piezoelectric transformer and the load must be connected 1:1. The striking voltage required to start the lamp is thus doubled, and the operating voltage for keeping the lamp on is also necessarily high.
An object of the present invention is therefore to provide a drive circuit for a small, high efficiency piezoelectric transformer with discrete primary and secondary sides (a balanced output piezoelectric transformer) to maintain constant cold cathode fluorescent lamp brightness by electrically connecting plural cold cathode fluorescent lamps connected in series to the secondary terminal of the balanced output piezoelectric transformer, and controlling the phase difference of the input and output voltages of the piezoelectric transformer.
A further object is to provide high reliability piezoelectric transformer elements by reducing the striking voltage and operating voltage.
A drive device for a cold cathode fluorescent lamp according to the present invention drives one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends, and comprises: a piezoelectric transformer having a pair of primary electrodes and first and second secondary electrodes, the piezoelectric transformer converting a primary ac input from the primary electrodes by a piezoelectric effect to a secondary ac output, outputting a secondary output in a first phase from the first secondary electrode and outputting a secondary output in a second phase opposite the first phase from the second secondary electrode, and enabling connection of the electrical terminals at both ends of the cold cathode fluorescent lamp between the one secondary electrode and the other secondary electrode; a drive arrangement for applying the primary ac input to the primary electrodes; and a brightness control circuit for controlling cold cathode fluorescent lamp brightness. The brightness control circuit detects a phase difference between the secondary ac output and primary ac input. When the detected phase difference is greater than a specified phase difference, the drive arrangement reduces the input power to the primary electrodes of the piezoelectric transformer to reduce the lamp brightness. When the detected phase difference is less than a specified phase difference, the drive arrangement increases the input power to the primary electrodes of the piezoelectric transformer to increase the lamp brightness. The detected phase difference is thus made equal to the specified phase difference.
This cold cathode fluorescent lamp drive device further preferably has a variable oscillation circuit for oscillating the primary ac input at a specified frequency; a startup control circuit for controlling the frequency of the primary ac input from the variable oscillation circuit to strike the cold cathode fluorescent lamp; and a startup detector for detecting cold cathode fluorescent lamp startup.
Yet further preferably, the startup control circuit controls the variable oscillation circuit to sweep the primary ac input from a specified frequency to a frequency below said frequency to strike the cold cathode fluorescent lamp, and controls the variable oscillation circuit to fix and oscillate at the frequency at which the startup detector detects cold cathode fluorescent lamp startup.
Yet further preferably, the brightness control circuit stops operating when striking the cold cathode fluorescent lamp.
Yet further preferably, the frequency of the primary ac input is a frequency other than a frequency at which the secondary side of the piezoelectric transformer shorts, and a frequency intermediate to the frequency at which the piezoelectric transformer secondary side shorts and the secondary side opens.
Yet further preferably, the primary ac input frequency is a frequency other than a frequency in the band xc2x10.3 kHz of the piezoelectric transformer resonance frequency when the secondary side shorts, and a frequency other than a frequency in the band xc2x10.3 kHz of the frequency intermediate to the resonance frequency of the piezoelectric transformer when the secondary side shorts and the resonance frequency when the secondary side is open.
Yet further preferably, the frequency of the primary ac input is higher than the frequency of the maximum step-up ratio of the piezoelectric transformer producing the lowest cold cathode fluorescent lamp load.
Yet further preferably, the cold cathode fluorescent lamp drive device additionally comprises an inductor connected in series with one primary electrode, forming a resonance circuit with the piezoelectric transformer. The drive arrangement comprises a dc power source, a drive control circuit for outputting a drive control signal based on the primary ac input frequency, and a drive circuit connected to the dc power source and both sides of the resonance circuit for amplifying the drive control signal to a voltage level required to drive the piezoelectric transformer, outputting the ac input signal to the resonance circuit, and inputting the ac voltage to the primary electrodes. The brightness control circuit comprises a voltage detector circuit for detecting the ac voltage of the secondary ac output from at least one of the secondary electrodes, and outputting an ac detection signal, a phase difference detector circuit for detecting a phase difference between the ac input signal and detected ac signal, and outputting a dc voltage according to the detected phase difference, a phase control circuit for controlling the phase of the drive control signal, and a comparison circuit for comparing the dc voltage and a reference voltage, and controlling the phase control circuit so that the dc voltage and reference voltage match.
Yet further preferably, the ac input signal frequency is near the resonance frequency of the resonance circuit.
Yet further preferably, the voltage detector circuit comprises: a level shifter for shifting the ac voltage of the secondary ac output to a specific voltage amplitude level; and a zero cross detection circuit for switching and outputting the ac detection signal when the level shifter output signal crosses zero.
Yet further preferably, the phase detector circuit comprises: a logical AND for taking the AND of the ac input signal and ac detection signal, and outputting a phase difference signal; and an averaging circuit for averaging the phase difference signal and outputting a dc voltage.
Yet further preferably, the drive circuit comprises: a first series connection having a first switching element and a second switching element connected in series; a second series connection parallel connected to the first series connection and having a third switching element and a fourth switching element connected in series; a first element drive circuit connected to the first switching element for driving the first switching element; a second element drive circuit connected to the second switching element for driving the second switching element; a third element drive circuit connected to the third switching element for driving the third switching element; and a fourth element drive circuit connected to the fourth switching element for driving the fourth switching element.
Yet further preferably, the resonance circuit is connected between the node between the first switching element and second switching element, and the node between the third switching element and fourth switching element.
In this case, the drive control signal preferably comprises: a first element control signal for driving the first element drive circuit; a second element control signal for driving the second element drive circuit; a third element control signal for driving the third element drive circuit; and a fourth element control signal for driving the fourth element drive circuit.
Yet further preferably in this case the first element control signal and second element control signal are controlled by the drive control circuit so that the first switching element and second switching element switch alternately on and off at a specific on time ratio; and the third element control signal and fourth element control signal are controlled by the drive control circuit so that the third switching element and fourth switching element switch alternately on and off at the same frequency and on time ratio as the first element control signal and second element control signal.
Yet further preferably, the first element control signal, second element control signal, third element control signal, or fourth element control signal is used in place of the ac input signal for phase difference signal detection.
Yet further preferably, the ac input signal is a rectangular signal combining the first element control signal, second element control signal, third element control signal, and fourth element control signal.
A cold cathode fluorescent lamp drive device according to a further aspect of this invention is a drive device for one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends, comprising: a piezoelectric transformer having a pair of primary electrodes and first and second secondary electrodes, the piezoelectric transformer converting a primary ac input from the primary electrodes by a piezoelectric effect to a secondary ac output, outputting a secondary output in a first phase from the first secondary electrode and outputting a secondary output of a second phase opposite the first phase from the second secondary electrode, and enabling connection of the electrical terminals at both ends of the cold cathode fluorescent lamp between the first secondary electrode and the second secondary electrode; a variable oscillation circuit for oscillating the primary ac input at a specified frequency; a drive arrangement for applying the primary ac input to the primary electrodes; and a brightness control circuit for controlling cold cathode fluorescent lamp brightness. The brightness control circuit detects the ac voltage of the secondary ac output applied to the end electrical terminals of the cold cathode fluorescent lamp. When the detected ac voltage of the secondary ac output is greater than a specific voltage, the brightness control circuit controls the variable oscillation circuit so that the primary ac input frequency approaches the resonance frequency of the piezoelectric transformer. When the detected ac voltage of the secondary ac output is less than the specific voltage, the brightness control circuit controls the variable oscillation circuit so that the primary ac input frequency recedes from the resonance frequency of the piezoelectric transformer. The detected ac voltage of the secondary ac output and the specific voltage thus become equal.
A cold cathode fluorescent lamp drive device according to a further aspect of this invention is a drive device for one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends, comprising: a piezoelectric transformer having a pair of primary electrodes and first and second secondary electrodes, the piezoelectric transformer converting a primary ac input from the primary electrodes by a piezoelectric effect to a secondary ac output, outputting a secondary output in a first phase from the first secondary electrode and outputting a secondary output of a second phase opposite the first phase from the second secondary electrode, and enabling connection of the electrical terminals at both ends of the cold cathode fluorescent lamp between the first secondary electrode and the second secondary electrode; a drive arrangement for applying the primary ac input to the primary electrodes; and a brightness control circuit for controlling cold cathode fluorescent lamp brightness. The brightness control circuit detects the ac voltage of the secondary ac output. When the detected ac voltage of the secondary ac output is greater than a specific voltage, the brightness control circuit controls the drive arrangement to lower the ac voltage of the primary ac input. When the detected ac voltage of the secondary ac output is less than a specific voltage, the brightness control circuit controls the drive arrangement to increase the ac voltage of the primary ac input. When the detected ac voltage of the secondary ac output is less than the specific voltage, the brightness control circuit controls the variable oscillation circuit so that the primary ac input frequency recedes from the resonance frequency of the piezoelectric transformer. The detected ac voltage of the secondary ac output and the specific voltage thus become equal.
A cold cathode fluorescent lamp device according to a further aspect of the invention has a cold cathode fluorescent lamp drive device according to the present invention, and one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends connected between the first and the second secondary electrodes of the piezoelectric transformer.
A drive method for a cold cathode fluorescent lamp according to the present invention is a method for driving one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends, comprising: applying a primary ac input from a drive arrangement to primary electrodes of a piezoelectric transformer, the piezoelectric transformer having a pair of primary electrodes and first and second secondary electrodes, the piezoelectric transformer converting the primary ac input from the primary electrodes by a piezoelectric effect to a secondary ac output, outputting a secondary output in a first phase from the first secondary electrode and outputting a secondary output in a second phase opposite the first phase from the second secondary electrode; striking the cold cathode fluorescent lamp connected with both end electrical terminals thereof connected between the first and the second secondary electrodes by applying the first phase secondary ac output to one of the electrical terminals, and applying the second phase second ac output to the other electrical terminal; detecting a phase difference between the secondary ac output and primary ac input by means of a brightness control circuit for controlling cold cathode fluorescent lamp brightness; controlling the drive arrangement to reduce primary ac input power to the primary electrodes of the piezoelectric transformer when the detected phase difference is greater than a specified phase difference; controlling the drive arrangement to increase primary ac input power to the primary electrodes of the piezoelectric transformer when the detected phase difference is less than a specified phase difference; and making the detected phase difference equal to the specified phase difference.
Preferably, a variable oscillation circuit for oscillating the primary ac input is controlled to sweep the primary ac input from a specified frequency to a frequency below said frequency to strike the cold cathode fluorescent lamp, and is then controlled to fix and oscillate at the frequency at which cold cathode fluorescent lamp startup is detected.
Further preferably, the frequency of the primary ac input is a frequency other than a frequency at which the secondary side of the piezoelectric transformer shorts, and a frequency intermediate to the frequency at which the piezoelectric transformer secondary side shorts and the secondary side opens.
Yet further preferably, the primary ac input frequency is a frequency other than a frequency in the band xc2x10.3 kHz of the piezoelectric transformer resonance frequency when the secondary side shorts, and a frequency other than a frequency in the band xc2x10.3 kHz of the frequency intermediate to the resonance frequency of the piezoelectric transformer when the secondary side shorts and the resonance frequency when the secondary side is open.
Yet further preferably, the frequency of the primary ac input is higher than the frequency of the maximum step-up ratio of the piezoelectric transformer producing the lowest cold cathode fluorescent lamp load.
A drive method for driving one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends according to a further aspect of the invention comprises: applying a primary ac input oscillated by a variable oscillation circuit from a drive arrangement to primary electrodes of a piezoelectric transformer, the piezoelectric transformer having a pair of primary electrodes and first and second secondary electrodes, the piezoelectric transformer converting the primary ac input from the primary electrodes by a piezoelectric effect to a secondary ac output, outputting a secondary output in a first phase from the first secondary electrode and outputting a secondary output in a second phase opposite the first phase from the second secondary electrode; striking the cold cathode fluorescent lamp connected with both end electrical terminals thereof connected between the first and the second secondary electrodes by applying the first phase secondary ac output to one of the electrical terminals, and applying the second phase second ac output to the other electrical terminal; detecting an ac voltage of the secondary ac output applied to the end electrical terminals of the cold cathode fluorescent lamp by means of a brightness control circuit for controlling cold cathode fluorescent lamp brightness; controlling the drive arrangement to reduce the ac voltage of the primary ac input when the detected ac voltage of the secondary ac output is greater than a specified voltage; controlling the drive arrangement to increase the ac voltage of the primary ac input when the detected ac voltage of the secondary ac output is less than a specified voltage; and making the detected ac voltage of the secondary ac output equal to the specified voltage.
A drive method for driving one or a plurality of series-connected cold cathode fluorescent lamps having an electrical terminal at both ends according to a yet further aspect of the invention comprises: applying a primary ac input oscillated by a variable oscillation circuit from a drive arrangement to primary electrodes of a piezoelectric transformer, the piezoelectric transformer having a pair of primary electrodes and first and second secondary electrodes, the piezoelectric transformer converting the primary ac input from the primary electrodes by a piezoelectric effect to a secondary ac output, outputting a secondary output in a first phase from the first secondary electrode and outputting a secondary output in a second phase opposite the first phase from the second secondary electrode; striking the cold cathode fluorescent lamp connected with both end electrical terminals thereof connected between the first and the second secondary electrodes by applying the first phase secondary ac output to one of the electrical terminals, and applying the second phase second ac output to the other electrical terminal; detecting an ac voltage of the secondary ac output applied to the end electrical terminals of the cold cathode fluorescent lamp by means of a brightness control circuit for controlling cold cathode fluorescent lamp brightness; controlling the variable oscillation circuit so that the primary ac input frequency approaches the resonance frequency of the piezoelectric transformer when the detected ac voltage of the secondary ac output is greater than a specific voltage; controlling the variable oscillation circuit so that the primary ac input frequency recedes from the resonance frequency of the piezoelectric transformer when the detected ac voltage of the secondary ac output is less than the specific voltage; and making the detected ac voltage of the secondary ac output and the specific voltage equal.
Yet further preferably, the primary ac input comprises the pulse signals of a plurality of switching elements driven by pulse signals, and the primary ac input is applied to the primary electrodes; and phase difference detection by the brightness control circuit detects a phase difference between pulse signals input to the switching elements, and the secondary ac output converted to a rectangular wave pulse signal by zero cross detection.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.